Oxygen generating device

ABSTRACT

An oxygen generating device to which a gas mixture containing oxygen is fed, and which is operable to separate the gas mixture into oxygen rich and oxygen depleted gas components, the device comprising a first material which when at an elevated temperature at or above a minimum operating temperature, is active to separate the gas mixture into oxygen rich and oxygen depleted gas components wherein a second material is provided which produces a heating effect within the device when an electrical current passes therethrough, to heat the gas mixture at least when the gas mixture is at a temperature below the minimum operating temperature.

DESCRIPTION OF BACKGROUND TO THE INVENTION

This invention relates to an oxygen generating device and to a systemincorporating an oxygen generating device.

In order for passengers or aircrew in an aircraft to breath when exposedto ambient atmospheric pressure at elevated altitudes, it is necessaryto provide a supply of breathing gas enriched with oxygen.

One means of achieving this is to carry within the airframe a supply ofcompressed oxygen gas, but particularly in a small aircraft, where spaceis at a premium, and/or in an aircraft where the added weight of the gasbottle containing the compressed oxygen gas is significant, this is notacceptable.

To reduce weight and space requirements another means is to carry withinthe airframe liquid oxygen. Liquid oxygen systems give rise to space andweight penalties and also a requirement for liquid oxygen to beavailable for replenishment of the liquid oxygen supply at a groundstation.

DESCRIPTION OF THE PRIOR ART

More recently oxygen-enriched gas has been produced on-board of theaircraft by a so-called on-board oxygen generating system (OBOGS) basedon pressure swing technology using a zeolite molecular sieve material toseparate oxygen from air. This requires at least two zeolite beds whichhave to be sequentially cycled through on-stream/generating andoff-stream/purge cycles. A limitation of such systems is thattheoretically the maximum oxygen concentration obtainable in the productgas is 95% unless additional means are provided for the removal of argonand other trace gases from the supply air which is normally bleed airfrom a compressor stage of an engine powering the aircraft.

Increasing attention is now being given to ceramic membrane technologyin provision of a system which will generate substantially 100% oxygenproduct gas or highly oxygen-enriched product gas of breathable qualityfor use in aerospace and other breathing applications. Such gas willhereinafter be referred to as being "oxygen rich", and the residual gas,will be referred to as being "oxygen depleted".

Certain ceramic materials (for example Yttria doped Zirconia or Gaddiadoped Ceria), which are so-called ionic conductors of oxygen, becomeelectrically conductive at elevated temperatures due to the mobility ofoxygen ions within the crystal lattice. Since these materials are onlyconductive to oxygen ions in the presence of an electric current, anexternal electric circuit is needed. Temperatures in the order of atleast 600° K. are required to obtain sufficient ionic conductivity.

Such ceramic oxygen generating devices may comprise one or more ceramicmembranes through which an electrical current is passed, whilst ambientair is supplied to one face of the membrane which allows oxygen in thesupply air to diffuse through the membrane by ionic transport when themembrane is at the required elevated temperature, and be recovered onthe other side of the membrane.

To ensure that the ambient air entering the device does not cool themembrane and prevent it from efficiently being heated to a minimumoperating temperature, the ambient supply air is pre-heated typically bypassing the supply air through a heat exchanger to which hot oxygen richgas and/or hot oxygen depleted gas delivered from the oxygen generatingdevice is fed, so that the cooler ambient air is heated.

The electrical current passing through the material has a heating effecton the material and on the air passing through the device, and at leastafter an initial warm up period, the oxygen generating device is selfsustaining at a temperature above the minimum operating temperature atwhich air is separated into its oxygen rich and oxygen depleted aircomponents.

The ambient air fed to the oxygen generating device, and electricalcurrent passed through the material can be controlled as necessary, toensure that the demand for the oxygen rich air supply is met, and sothat overheating of the oxygen generating device does not occur.

However, whereas such oxygen generating devices operate generallysatisfactorily after an initial warm up period, there is a requirementto reduce the warm up period to a minimum time. In any event, with atleast some commonly used ceramic membrane materials, there is a highelectrical resistance through the material at lower temperatures, and sothe magnitude of electrical current which can pass through the materialto cause a heating effect, is small during the initial warm up periodwhen the oxygen generating device is at a low temperature. Hence littleor no heat is dissipated to the air passing through the oxygengenerating device.

Hence for rapid warm up it is necessary to provide a preheater to heatthe ambient air prior to the ambient air passing through the oxygengenerating device. Such a preheater may comprise a conventionalelectrical resistance heater, but due to the magnitude of currentrequired, bulky and heavy switch and control gear is required, which hasa space requirement and significant weight implications.

SUMMARY OF THE INVENTION

According to a first aspect of the invention we provide an oxygengenerating device to which a gas mixture containing oxygen is fed, andwhich is operable to separate the gas mixture into oxygen rich andoxygen depleted gas components, the device comprising a first materialwhich is active at an elevated temperature above a minimum operatingtemperature, to separate the gas mixture into oxygen rich and oxygendepleted gas components, there being a second material provided whichproduces a heating effect within the device when an electrical currentpasses therethrough to heat the gas mixture at least when the gasmixture is at a temperature below the minimum operating temperature.

Thus there is no need to provide any separate preheating means toprovide for rapid warm up of the oxygen generating device. The secondmaterial may comprise a positive temperature coefficient of resistancematerial, the heating effect of which may decrease as the temperaturewithin the device increases towards the minimum operating temperature.The first material may comprise a negative temperature coefficient ofresistance material and electrical current may be passed through thefirst material so that this provides a heating effect within the deviceat least when the device is at a temperature at or above the minimumoperating temperature.

Hence on start-up, the second material will conduct the electric currentthus to provide a heating effect to the gas mixture passing through theoxygen generating device. As the device warms up, the heating effect ofthe second material may decrease, but the heating effect of the firstmaterial will increase, and the first material can then become active tocause separation of the oxygen rich and oxygen depleted gas componentsof the gas mixture passing into the device.

The gas mixture may pass into contact with one of the first and secondmaterials, and then subsequently into contact with the other of thefirst and second materials, the first and the second materials beingarranged separately from one another within the device. Alternatively,the first and second materials may be contained within a matrix suchthat the gas mixture may pass simultaneously into contact with both thefirst and second materials.

One embodiment of the oxygen generating device may comprise a housinghaving a passage therethrough for gas in which passage the first andsecond materials are arranged. The gas may pass through layers of thefirst and second materials there being duct means into which the oxygenrich gas component can flow and hence be collected for use.

In another embodiment, the device may comprise one or more passages forgas through the device, defined by passage walls which support the firstand second materials, such that the gas contacts the first and secondmaterials as it flows through the passage or passages. In this case, thewalls of the passages preferably permit oxygen rich gas component topass out of the passage or passages, through the walls to a duct meansfrom which the oxygen rich gas component can be collected for use.

According to a second aspect of the invention, we provide an oxygengenerating system including an oxygen generating device according to thefirst aspect of the invention, the system including a heat exchangerthrough which the gas mixture passes prior to being fed to the oxygengenerating device, there being passage means for heated oxygen depletedgas component and/or oxygen rich gas component from the oxygengenerating device to be fed to the heat exchanger such that the gasmixture is preheated by heat exchanged from the oxygen depleted gascomponent and/or the oxygen rich gas component.

The system may include a control means to control the gas mixture fed tothe oxygen generating device and to control the temperature within thedevice thereby to achieve a desired rate of production of oxygen richgas component.

According to a third aspect of the invention we provide an oxygengenerating device to which a gas mixture containing oxygen is fed, andwhich is operable to separate the gas mixture into oxygen rich andoxygen depleted gas components, the device comprising a negativetemperature coefficient of resistance material which is active at anelevated temperature above a minimum operating temperature, to separatethe gas mixture into oxygen rich and oxygen depleted gas components,there being means to pass an electrical current through the device toproduce a heating effect within the device to heat the gas mixture atleast towards the minimum operating temperature, wherein the devicecomprises a plurality of active sections through each of which the gasmixture passes in turn and the electrical current supply is connected toeach of the sections so that the sections electrically are in series.

Thus the electric current passing through any of the active sections, islimited by the active section with the highest electric resistance,which in general will be the coolest section.

Particularly where heated oxygen enriched and/or oxygen depleted gasfrom the device is used to preheat the gas mixture fed to the device,the device can be thermally managed, at least when the temperature ofthe device is above the minimum operating temperature, by a less complexthermal control system than would otherwise be required, to ensure thatoverheating of the device does not occur.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is a diagrammatic illustration of an oxygen generating systemincorporating an oxygen generating device in accordance with theinvention;

FIG. 2 is an illustrative sectional view of one embodiment of an oxygengenerating device in accordance with the invention;

FIG. 3 is a diagrammatic exploded view of the embodiment of FIG. 2showing the respective, air, current and oxygen paths through and in thedevice;

FIG. 4 is a diagrammatic illustrative view in cross-section to anenlarged scale of part of the oxygen generating device of FIG. 2;

FIG. 5 is a diagrammatic illustrative view on an alternate embodiment;

FIG. 6 is a graph showing the relative conductivities of first andsecond materials which may be used within an oxygen generating device inaccordance with the invention, as conductivity varies with temperature;

FIG. 7 is a diagrammatic illustrative perspective view of anotherembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown an oxygen generating system 10,which has at its heart, a ceramic membrane module 11 being an oxygengenerating device in accordance with the invention.

A gas mixture such as for example ambient air from an uncompressedcompartment of an aircraft, enters the system through an inlet 12, whereany debris entrained in the inlet 12 is filtered from the ambient air bya filter 13. The air is drawn into the inlet 12 and is fed through theremainder of the system via a fan 14 the speed of which is controlled byan electrical control unit 15 as hereinafter described.

The ambient air which may be at a very cool temperature indeed, possiblybelow 273° K. then passes through a heater module 16 which will bedescribed in more detail hereinafter, where, at least after the system10 has warmed up, the ambient air will be pre-heated before the airpasses into the ceramic membrane module 11.

The ceramic membrane module 11 generates oxygen rich air component ashereinafter described, which passes from the module 11 via an outlet 17.The oxygen rich air component passes through the heater module 16 vialine 17a where at least some of its heat is dissipated to the ambientair, so that a cooled oxygen rich air supply is obtained, which is fedvia line 18a to a plenum 18, where the oxygen rich gas component maypass to, for example, an aircrew via a filter 19 where the oxygen richair component can be breathed, particularly at ambient atmosphericpressure at elevated altitudes.

The volume of the plenum 18 may be adjusted as required, but generallyis of a small volume being the equivalent to perhaps, the volume of line18a from the heater module 16 to the filter 19.

The ceramic membrane module 11 also produces a supply of hot oxygendepleted air component which passes from the module via an outlet 20.The hot oxygen depleted air component is also fed to the heater module16, along a line 20a. where at least some of its heat is dissipated tothe ambient air passing through the heater module 16. The cooled oxygendepleted air component then passes from the heater module 16 via line16b and is disposed of, for example through an external port 16c of theaircraft.

Conventionally, the heater module 16 would also comprise an electricalresistance or some other kind of auxiliary heater so that during aninitial warm up period, the ambient air entering the heater module 16can be warmed so that warmed air is fed to the ceramic membrane module11 rather than cold air and this is still an option in combination withthe present invention.

It will be appreciated from the discussion below that the ceramicmembrane module 11 can only operate to separate the ambient air into itsoxygen rich and oxygen depleted air components, when at a temperatureabove a minimum operating temperature, typical operating temperaturesare in the range 800°-1200° K. In accordance with the invention,however, such auxiliary heating in the heater module 16 is not required.However, it will be appreciated from FIG. 1 that the temperature of theambient air within the heater module 16, and/or the temperature of theoxygen rich and/or oxygen depleted air component within the heatermodule 16, is monitored, so as to provide a suitable input via line 23to the electronic control unit 15. Also, the temperature within theceramic membrane module 11 is also monitored, so as to provide an input24 to the electronic control unit 15 to protect the ceramic membranemodule 11 from overheating.

The pressure of the oxygen rich air component supply in plenum 18 isalso monitored, e.g. by a pressure transducer which provides an input 25to the electronic control unit 15. The speed of the fan 14, and hencethe volume of air being delivered to the heater module 16 andsubsequently the ceramic membrane module 11, is monitored and an input26 is provided to the electronic control unit 15.

In response to demand for oxygen rich gas component the electroniccontrol unit 15 controls the speed of the fan 14 via a line 27, and thepower fed to the ceramic membrane module 11 via a line 28 to control thelevel of oxygen generation in the ceramic membrane module 11. There isalso a built in test which results in an output indicated at 29 forexample, to alert an aircrew to the fact that the system 10 is notoperating correctly.

Operation of the ceramic membrane module 11 will now be described inmore detail with reference to FIGS. 2 to 6.

Referring first to FIG. 2, the ceramic membrane module 11 comprises ahousing 30 which has a passage 35 for gas therethrough. The passage hasan inlet 31, a first flow reverse box 32, a second flow reverse box 33,and an outlet 20 to which oxygen depleted air component is fed from thedevice 11 via ducting 34.

The ceramic membrane module 11 comprises a first, ceramic, materialthrough which electrical current is passed, the material being activewhen at an elevated temperature above a minimum operating temperature tocause separation of air or other gas into oxygen rich and oxygendepleted gas components. Generally, the oxygen rich gas component willflow orthogonally to the oxygen depleted gas component, and both suchcomponents will flow in directions generally orthogonal to the directionin which the electrical current is flowing.

In FIG. 2, in a first passage part 35a between the inlet 31 and thefirst flow reverse box 32, there is a first active section A comprisingfirst material 36a in the form of a ceramic membrane stack. One suitablematerial consists of an electrolyte membrane such as Cerium GadoliniumOxide (CGO) coated on both sides with an electrode made for example ofLanthanum Strontium Cobalt Ferrite (LSCF). The direction of flow of theoxygen depleted air component is indicated at 37 (which is the samedirection as the gas mixture), whilst the direction of flow of oxygenrich air component is indicated at 38. It can be seen that direction 38is generally orthogonal to the extent of the first passage part 35abetween the inlet 31 and the first flow reverse box 32, such that theoxygen rich air component flows into a duct 39 from which it may becollected and flow from the device 11 to the outlet 17 shown in FIG. 1.

The oxygen depleted air component flow direction is reversed in box 32,and passes into a second active section B which comprises a passage part35b between the flow reverse boxes 32 and 33. The second passage part35b contains a membrane stack 36b of first material, through which thegas flows. Again, the direction of flow of the oxygen depleted componentis indicated at 37, and the directions of flow of the oxygen rich aircomponent into duct 39 are indicated at 38.

Within a third passage part 35c between the second flow reverse box 33and the ducting 34 for the oxygen depleted air component, there is athird active section C comprising yet another membrane stack 36c offirst material. Again the relative directions of flow of the oxygendepleted air component and oxygen rich air component through membranestack 36c are indicated by arrows 37 and 38 respectively.

Referring now to FIG. 6, there is shown in full lines a graph showinghow the conductivity of the first material 36a-36c changes withtemperature. Below a cut-off temperature t_(min) it can be seen that thefirst material 36a-36c provides substantially no conductivity. The firstmaterial 36a-36c will fail to separate the ambient air into its oxygenrich and oxygen depleted air components until the material 36a-36c is ata minimum operating temperature t_(mot).

Hence during an initial warm up period, little or no heat may bedissipated to the ceramic membrane module 11 by the first material36a-36c as little or no electrical current will be permitted to flowtherethrough because the first material 36a-36c will be substantiallynon-conducting.

In accordance with the invention, prior to the air passing through thefirst material 36a in the first passage part 35a, the air passes througha membrane stack 40a of a second ceramic material. The second material40a may again be a ceramic material, but exhibiting a positivetemperature coefficient of resistance. Suitable second material may bebased on the barium titanate perovskite system.

Hence at temperatures below t_(min) indicated in FIG. 6, the secondmaterial 40a will conduct electricity therethrough and thus a heatingeffect will be achieved in the ceramic membrane module 11. In FIG. 6,the conductivity of the second material 40a relative to temperature isindicated by the dotted lines. As the temperature within the module 11increases, the amount of electrical current conducted by the secondmaterial 40a decreases and hence the heating effect due to the material40a will decrease. It can be seen that it is envisaged that theconductivities of the first and second materials 36a-36c and 40a will beabout equal around the temperature t_(mot) being the minimum operatingtemperature, which the first material 36a-36c must attain in order toperform its function of separating the ambient air into oxygen rich andoxygen depleted air components. This may be around 600° K.

Referring again to FIG. 2, in the second passage part 35b between thefirst flow reverse box 32 and the second flow reverse box 33, there is afurther membrane stack of second material 40b, and in the third passagepart 35c between the second flow reverse box 33 and the duct 34, thereis a yet further membrane stack of second material 40c. In thisembodiment, the air as it flows through the ceramic membrane module 11sequentially comes into contact with second material 40a, then firstmaterial 36a and so on. In another embodiment, instead of the first andsecond materials 36a-36c and 40a-40c being arranged in membrane stacksseparate from one another as indicated in FIG. 2, the materials may becontained within a common matrix such that air passing through theceramic membrane module 11 may simultaneously come into contact withinthe first and second material contained in the matrix.

The amount of oxygen rich air component generated, can be adjusted byregulating the electrical current which passes through the ceramicmembrane module 11, e.g. by changing the voltage across the module 11,and by adjusting the rate of delivery of ambient air to the inlet 31, byadjusting the speed of fan 14.

In the event that the temperature within the ceramic membrane module 11increases to such an extent that the oxygen rich gas component is attoo-elevated a temperature e.g. for comfortable breathing, which will beapparent by monitoring the temperature within the ceramic membranemodule 11, the amount of ambient air fed to the ceramic membrane module11 may be increased by increasing the speed of fan 14, and/or, theamount of electrical current passing through the device 11 may bedecreased by reducing the voltage across the device.

Thus the speed of the fan 14 may be determined with reference to theinput 25 from the plenum 18, and/or in dependence upon a feedback signalderived from the temperature of the ceramic membrane module 11 and/orthe temperature of oxygen rich gas components generated therein or theoxygen depleted air.

Referring now to FIG. 3 the device of FIG. 2 is shown diagrammaticallywith the paths of air flow and oxygen rich gas flow being indicated at37 and 38 respectively. Other parts are labelled with the same referencenumerals as in FIG. 2.

It can be seen from FIG. 3 that the electrical current is arranged topass through the three ceramic stack active sections A, B and C inseries so that there is a voltage drop across each active section A, B,C.

It has been found that utilising a single sheet membrane of firstmaterial which is active to separate the gas mixture into oxygen richand oxygen depleted components is unacceptable in design terms andimpracticable due to the required size of that sheet. Also theelectrical current flow required through such a membrane to achieveadequate volumes of separation of oxygen rich and oxygen depleted gascomponents, is unacceptably high.

Hence the preferred design of the device is to provide a plurality ofseparate stacks of active first (and second) material. Thus apractically sized device 11 can be provided and acceptably lowelectrical current is required.

Nevertheless there remains a problem of thermal gradients within thedevice, which can result in thermal stresses which could seriouslydamage the integrity of the device.

In accordance with the third aspect of the invention however theelectrical current is caused to flow in series through the activesections A, B and C of the device. In this way it will be appreciatedthat the electrical resistance of the second active section B, the firstmaterial 36b of which is a negative temperature coefficient ofresistance material, is dependent upon the temperature of the gasmixture (air) delivered to it from the first active section A of thedevice 11.

Hence the inventors realised that the device 11 can thermally be managedwith a control system less complex than would otherwise be required. Asthe temperature of air entering active section B of the device 11increases, the resistance of the first negative temperature coefficientof resistance material 36b in active section B will decrease resulting areduced heating effect in active section B with the result that thetemperature rise of the oxygen depleted air fed through active section Bof the device to active section C, will be restricted. Any increase intemperature of the air fed to active section C, could for example by adecrease in the temperature of the air fed to active section B, willcause the heating effect of the negative temperature coefficient ofresistance first material 36c of the third active section C of thedevice to decrease, and so the heating effect of active section C andany further active sections of the device 11 will be restricted.

Hence by connecting the active sections A, B and C of the device, or atleast two active sections of the device in series, the device 11 canthermally be managed at least when operating at a temperature above theminimum operating temperature of the first material 36a to 36c.

Although in the example described the first active section A is thecoolest and has the higher resistance, in another arrangement, anotherof the active sections B, C could be arranged to have the highestelectrical resistance and thus be primarily responsible for limiting theelectric current through all the active sections A, B and C.

In FIG. 4 a portion of one of the active section A of the device 11 isshown illustratively to an enlarged scale.

Passages P1-P3 for the air and oxygen depleted gas are formed bycastellations of a corrugated or castellated interconnect sheets Y1 toY3 and perforated sheets C1 to C3 being respective cathodes of thedevice 11 for each of the individual active layers of the stack section.Each cathode C1 to C3 forms one layer of a tri layer T1,T2,T3 whichcomprises the planar cathode C1 to C3, an intermediate planar electrodelayer E1 to E3, which is of a first, negative temperature co-efficientof resistance material 36a and a planar anode A1 to A3.

A voltage is applied across the device between the interconnect sheet R1and a boundary sheet R4. R2 and R3 also act as interconnect sheets.

Oxygen molecules contained in the gas stream within the passages P1 toP3 diffuse through the porous cathode layers C1 to C3 to the electrolytelayers E1 to E3. The applied voltage causes the oxygen molecules toionise at the electrode layers E1 to E3. The resultant anions passthrough the electrolyte layers E1 to E3 and reform at the anode layersA1 to A3, and hence pass into the passages Q1 to Q3 which extendtransversely to the passages P1-P3 and are also formed bycorrugations/castellations formed in a corrugated or castellated sheetZ1-Z3. Electrical current thus passes through the active section A ofthe device in the direction shown by arrow F through the various layersof the active section A of the device 11.

The construction shown in FIG. 4 shows a device with three layers,although more than three layers may be provided.

For an air pre-heating section, the arrangement may be similar to thatof FIG. 4, but the electrolyte layers E1 to E3 would be replaced bysuitable positive temperature co-efficient of resistance material (40a).The electrical potential applied across the outermost interconnect andboundary sheets R1 and R4 allows electrical current to flow through thevarious layers of the section of the device, when the temperature isbelow that required for minimum operation.

Whilst such a preheating section could be separate from the activeoxygen generating section A, advantageously the sections are combined,with the sections being separated from one another by separators, suchas indicated at D1 to D3 in FIG. 4.

Thus, a flow of electrical current can occur through the preheatingsection (or sections) and through the oxygen generating section (orsections) depending on the temperature, as indicated by the graph ofFIG. 6.

By virtue of the preheating section or sections the onset of oxygenproduction occurs on start up, before it otherwise would. The transitionbetween electrical current passing through the preheating section orsections and the oxygen generating section or section would ideally beinstantaneous, but practically there is a transition zone where there isparallel electrical conductivity, as indicated in the graph of FIG. 6,which occurs with both the preheating and oxygen generating sectionsbeing conductive.

Various alternative constructions to that shown in FIGS. 2 to 4 arepossible in which air passes separately through active sections, towhich electrical current is applied, to pass in series through thesections. For example, the same principles can be applied to a tubularceramic oxygen generating arrangement.

In FIG. 5 a flat plate design of one active section A of a ceramicmembrane module 211 is shown, there being a first electrode 250 and asecond electrode 251, with a matrix containing first and second material236 and 240 between them. Ambient air passes into passageways providedbetween the lower plate electrode 250 and the matrix 236/240 of ceramicmaterials, whilst electrical current flows in the direction shown byarrow G, between one electrode 251 and the other electrode 250.

Oxygen depleted air component flows from the passageways 235 in thedirection indicated by arrow D, whilst oxygen rich gas component flowsfrom passageways in the direction shown in arrow E from the device, foruse.

A plurality of such plate constructions may be arranged stacked one ontop of another to provide each active section.

Various other alternative constructions are possible without departingfrom the scope of the invention. For example, referring to FIG. 7, thereis shown a device 311 which is essentially cylindrical in construction,having a central passageway 335 from one end, being an inlet end 331, toan outlet end 320. The device 311 is made of a suitable ceramicmaterial, which is porous in nature. Porous passage walls support in thepassageway 335, within a first part 312, second material 340 which has anegative temperature coefficient, like the material 40a-40c of themodule 11 described with reference to FIG. 2.

The porous walls support within the passageway 335 along a second part343 of the device 311, first material 336 of the kind indicated at36a-36c in the module 11 of FIG. 2, electrical current being passedthrough walls of the device between an external electrode indicated at350, and an internal electrode 351, with the active first or secondmaterial positioned between the electrodes.

During an initial warm up period, the second material in the first part312 of the device will dissipate heat to ambient air entering thepassage 335 via the inlet 331 thus to heat the air. As the temperatureof the device 311 increases, the second material within the second part313 of the device will become activated to separate the ambient air intooxygen depleted air component which flows from outlet 320 of the device311, and oxygen rich air component which will flow through the porouswalls of the device 311 into a duct means (not shown) from where it maybe collected for use.

Hence various modifications are possible without departing from thescope of the invention, the essential components of the first and secondaspects of the invention being the presence of a second material whichcan produce a heating effect within the oxygen generating device whenelectrical current passes through the material to heat the gas mixture,at least when the gas mixture within the device is at a temperaturebelow the minimum operating temperature of the first material, andpossibly at temperatures also below the temperature at which the firstmaterial can conduct electricity and thus dissipate heat within theoxygen generating device. Thus the first material may not be of the kindthrough which electrical current is passed to produce a heating effect,but the gas passing into the device may be heated by other means toabove the minimum operating temperature.

The actual arrangement of control of an oxygen generating systemincorporating such a device may be modified compared to the arrangementshown in FIG. 1 which is given for example only. The relationship ofconductivity to temperature for actual materials selected for use maynot be exactly as indicated in FIG. 6.

We claim:
 1. An oxygen generating device which is operable to separategas mixture fed thereto into oxygen rich and oxygen depleted gascomponents, the device comprising an inlet means to which a gas mixturecontaining oxygen is fed, first material which is active at an elevatedtemperature above a minimum operating temperature, to separate the gasmixture into oxygen rich and oxygen depleted gas components, and secondmaterial which produces a heating effect within the device when anelectrical current passes therethrough to heat the gas mixture at leastwhen the gas mixture is at a temperature below the minimum operatingtemperature.
 2. A device according to claim 1 wherein the secondmaterial comprises a positive temperature coefficient of resistancematerial, the heating effect of which thus decreases as the temperaturewithin the device increases towards the minimum operating temperature.3. The device according to claim 1 wherein the first material comprisesa negative temperature coefficient of resistance material which providesa heating effect within the device when electrical current passestherethrough at least when the temperature is at or above the minimumoperating temperature.
 4. A device according to claim 1 wherein the gasmixture passes into contact with one of the first and second materialsand subsequently into contact with the other of the first and secondmaterials, the first and second materials being separate from oneanother within the device.
 5. A device according to claim 1 wherein thefirst and second materials are contained within a matrix such that thegas mixture may pass simultaneously into contact with the first andsecond materials.
 6. A device according to claim 1 which comprises ahousing having a passage therethrough for the gas in which passage thefirst and second materials are arranged, the first and second materialsbeing arranged in layers such that the gas passes through the first andsecond materials and there being duct means into which the oxygen richgas component can flow and hence be collected for use.
 7. A deviceaccording to claim 1 which comprises at least one passage for gasthrough the device defined by passage walls which support the first andsecond materials such that the gas contacts the first and secondmaterials as it flows through the passage or passages.
 8. A deviceaccording to claim 7 wherein the walls of the passage permit oxygen richgas component to pass out of the passage to a duct means from which theoxygen rich gas component can be collected for use.
 9. An oxygengenerating system including an oxygen generating device which isoperable to separate gas mixture fed thereto into oxygen rich and oxygendepleted gas components, the device comprising an inlet means to which agas mixture containing oxygen is fed, first material which is active atan elevated temperature above a minimum operating temperature, toseparate the gas mixture into oxygen rich and oxygen depleted gascomponents, and second material which produces a heating effect withinthe device when an electrical current passes therethrough to heat thegas mixture at least when the gas mixture is at a temperature below theminimum operating temperature, the system including a heat exchangerthrough which the gas mixture passes prior to being fed to the oxygengenerating device, there being passage means for at least one of heatedoxygen depleted gas component and oxygen rich gas component from theoxygen generating device to be fed to the heat exchanger such that thegas mixture is preheated by heat exchanged from the at least one of theoxygen depleted gas component and the oxygen rich gas component prior tobeing fed to the oxygen generating device.
 10. A system according toclaim 9 which includes a control means to control the gas mixture fed tothe oxygen generating device and to control the temperature within thedevice thereby to achieve a desired rate of producing of oxygen rich gascomponent, with a required proportion of oxygen contained therein.
 11. Aoxygen generating device to which a gas mixture containing oxygen isfed, and which is operable to separate the gas mixture into oxygen richand depleted gas components, the device comprising a negativetemperature coefficient of resistance material which is active at anelevated temperature above a minimum operating temperature, to separatethe gas mixture into oxygen rich and oxygen depleted gas components,there being means to pass an electrical current through the device toproduce a heating effect within the device to heat the gas mixture atleast towards the minimum operating temperature, wherein the devicecomprises a plurality of active sections through each of which gasmixture passes in turn and the electrical current supply is connected toeach of the sections so that the sections electrically are in series.